Following recent increasing demands of optical communications using plastic optical fibers such as LAN, etc., increase in the communication distance and enhancement of resistance to environments, particularly resistance to heat (which means that the transmission characteristic is not varied with respect to the temperature variation) have been required.
A plastic optical fiber having a core made of methyl methacrylate polymer has been widely used as an optical transmission path for optical communications because it has advantages such as low absorbance, etc. A red light-emitting diode is generally used in an optical communication apparatus using such a plastic optical fiber as an optical transmission path.
In the conventional optical communication apparatus using a red light-emitting diode and a plastic optical fiber having a core made of polymethylmethacrylate resin as described above, the wavelength of light emitted from a light source is liable to vary due to temperature variation, and the variation of the wavelength of the emitted light sharply increases transmission loss of the plastic optical fiber. Particularly in the case of a light-emitting element having a broad full width at half maximum of wavelength, the wavelength components other than those around 650 nm in wavelength are sharply attenuated. Therefore, the transmission loss is increased and thus it is difficult to perform long-distance optical communication (transmission). For example, the transmission of about 100 m at maximum can be performed by an optical communication apparatus using a plastic optical fiber which is currently on the market.
Recently, blue and green light-emitting diodes (LED) having high output power have been developed, and they have been expected to be used as light sources for optical communications. For example, JP(A)-8-116309 discloses that a blue light-emitting element is used as a light source for an optical communication apparatus from the viewpoint of the resistance to heat.
Since the optical communication apparatus disclosed in JP(A)-8-116309 uses the blue light-emitting element as a light source, the heat resistance of the light source itself is excellent. However, when this light source is used in combination with a plastic optical fiber, the heat resistance of the plastic optical fiber becomes lower.
That is, as disclosed in JP(A)-8-116309, the blue light-emitting element for emitting light having short wavelength has little effect on the light emission characteristic due to the temperature variation because it has a broad forbidden band width, and thus the heat resistance thereof is excellent. However, in the conventional plastic optical fiber, electron transition absorption due to thermal oxidation deterioration of the optical fiber more remarkably occurs to light having shorter wavelength, and thus the transmission loss is more greatly increased in the blue region.
Further, JP(A)-9-318853 discloses an optical transceiver for performing bi-directional communications through a single-core optical fiber, which uses a yellow light-emitting element for emitting light of 570 nm in wavelength and a plastic optical fiber having a core formed of polymethylmethacrylate. However, this optical transceiver carries out the bi-directional communications through a single core, and thus it does not aim at long-distance transmission. Therefore, it has a drawback that S/N is too low to carry out the long-distance optical transmission.
Still further, the optical communication apparatus disclosed in JP(A)-8-116309 and the optical transceiver disclosed in JP(A)-9-318853 are not suitable for long-distance transmission because the optical fibers used in these apparatuses are not suitable for optical transmission in a short-wavelength range such as blue, yellow, etc.